In an oil-filled electric device such as an oil-filled transformer or the like, copper coil serves as a conducting medium. The coil is wrapped by an insulating paper to ensure electric insulation, preventing the coil from short-circuiting electrically between adjacent turns.
Meanwhile, the oil-filled transformer is filled with an electric insulating oil generally containing mineral oil or the like. It has been known that the mineral oil or the like contains a very small amount of sulfur ingredient, and the sulfur ingredient reacts with copper coil disposed in the electric insulating oil to generate conductive copper sulfide on a surface of the insulating paper wrapped on the coil. It has been known that the generation of copper sulfide will reduce insulating performance of the insulating paper wrapped on the coil, and even cause the coil to short-circuit between turns, leading to dielectric breakdown (for example, NPL 1).
It has been known that a major causative substance for the generation of copper sulfide is dibenzyl disulfide serving as the sulfur ingredient in the mineral oil (for example, NPL 2). As a generation mechanism of copper sulfide , it has been known that after a complex generated from the reaction of dibenzyl disulfide with copper coil diffuses in the oil and adheres to the insulating paper, it decomposes to generate copper sulfide (for example, NPL 3).
It has been known that inhibiting, on the basis of the generation mechanism mentioned above, the reaction between dibenzyl disulfide and copper coil leads to preventing the generation of copper sulfide; therefore, methods of adding an inhibitor to the electric insulating oil have been widely employed. As the inhibitor, there has been known benzotriazole compound such as 1, 2, 3-benzotriazole (BTA), Irgamet 39 or the like (for example, NPL 4).
After the inhibitor against the generation of copper sulfide is added to the electric insulating oil, the inhibitor reacts with copper coil to form a film on the surface of copper coil. Owing to the formed film, the reaction between dibenzyl disulfide and copper coil is blocked or inhibited; thereby, the generation of copper sulfide can be prevented (for example, NPL 4).